This invention relates to a system and method for detecting a flaw in a sample and more particularly to such a system and method which distinguishes shear waves from the other acoustic waves to determine the presence and the location, size and orientation of a flaw.
Conventional flaw detection systems, particularly those used for examining railroad rails in situ employ a sonic generator such as a piezoelectric ultrasonic transducer in the hub of a wheel that rides on the rail. The acoustic energy is injected into the rail at an angle through the rolling wheel and the backscattered or reflected energy from the flaw is detected either by another transducer or by the same one in pulse echo mode. One shortcoming of this approach is that it requires contact with all the attendant problems of wear, damage, alignment and the inherent limitation on speed. Since these devices require a significant volume it is not always possible to use them to engage the side walls of the rail or sample.
It is therefore an object of this invention to provide an improved system and method for detecting flaws in a sample.
It is a further object of this invention to provide such a system and method which does not require contact with the sample under examination.
It is a further object of this invention to provide such a system and method which operates in any orientation about the sample.
It is a further object of this invention to provide such a system and method which employs small, compact and light apparatus.
It is a further object of this invention to provide such a system and method which is not inherently limited as to speed.
It is a further object of this invention to provide such a system and method which can determine size, location and orientation of a flaw.
It is a further object of this invention to provide such a system and method which rely on flaw shadows rather than backscattering and so are more sensitive.
It is a further object of this invention to provide such a system and method which can operate remotely from the sample using mirrors or optical fibers.
The invention results from the realization that since shear waves travel a different path and/or have a different velocity than longitudinal and surface Rayleigh waves the presence of a flaw in a sample can be detected by sensing energy levels of the shear waves in a time window that minimizes interference from longitudinal and surface Rayleigh waves, and the further realization that the presence and amplitude of surface Rayleigh waves can be used to normalize the shear wave flaw detection signals and that the shadows of a flaw that indicate the presence of a flaw can be further used to identify the size, location and orientation of that flaw.
This invention features a defect detection system including an excitation laser system for projecting a laser beam at the near surface of the sample to be tested for generating acoustic longitudinal, surface Rayleigh, and shear waves in the sample. A detection laser system spaced from the excitation laser intercepts shear waves reflected from the far surface of the sample at approximately the angle of maximum shear wave propagation. A detection circuit detects the energy level of the reflected shear wave intercepted by the detection laser system representative of a flaw in the sample.
In a preferred embodiment, the excitation laser system and the detection laser system may be on the same side of the sample. There may be a movable support for the excitation laser system and the detection laser system for moving them along the sample. The detection circuit may include a shear wave sensing circuit for sensing the energy level of the acoustic wave at the time of arrival of the reflected shear wave at the detection laser system. A detection circuit may also include a first logic circuit for recognizing the presence of a potential flaw if the energy level of the acoustic wave sensed by the shear wave sensing circuit is less than the predetermined level. The detection circuit may also include a surface Rayleigh wave sensing circuit for sensing the energy level of the acoustic wave at the time of arrival of the surface Rayleigh wave at the detection laser system. There may be a second logic circuit for inhibiting recognition of a potential flaw if the energy level of the acoustic wave sensed by the surface Rayleigh wave sensing circuit is less than the predetermined level and confirming recognition if it is greater than the predetermined level. The detection circuit may include a scanning device for sensing the variation and the energy level of the reflected shear wave along the sample to create shadows of a flaw. A measuring circuit may be used for measuring the length of each shadow cast by a flaw blocking shear wave propagation and the distance between those shadows. A positioning circuit may be used for determining the location, size and orientation of a flaw. The sample may include steel and the angle of maximum shear wave propagation may be approximately 40xc2x0.
The invention also features a method of detecting a defect in the sample including photoacoustically exciting acoustic longitudinal surface Rayleigh and shear waves at a first point on the near surface of the sample. Acoustic waves are then detected photoacoustically at a second point spaced from the excitation first point for intercepting shear waves reflected from the far surface of the sample at approximately the angle of maximum shear wave propagation. The energy level of the intercepted reflective shear wave is detected as being representative of a flaw in the sample.
In a preferred embodiment, the excitation and detection may occur on the same side of the sample. The excitation and detection points may be moved along the sample. The energy level of the reflected shear wave may be sensed and the presence of a potential flaw is recognized if the energy level is below a predetermined level. The energy level of the surface Rayleigh waves may be sensed and used to inhibit detection of a flaw if the level of the Rayleigh waves is below a predetermined level. If it is above a predetermined level, it may be used to confirm recognition of a flaw. The variations in energy level of the reflective shear wave along the sample may be used to create shadows of the flaw. The length of each of the shadows cast by the flaw may be measured and then location, size and orientation of the flaw may be determined from the size and the separation of those shadows.